Identification of chemicals in the environment that adversely affect reproductive capacity and/or development of wildlife populations and humans have become a major public health issue in recent years. The US Congress has responded to these concerns with legislation that mandates the government to investigate the effects of these environmental endocrine disruptor chemicals (EDCs). The EPA has proposed a two tiered system to identify EDCs and to assess their ability to adversely affect reproductive health. Tier-1 assays determine the ability of a chemical to disrupt hormone action at the cellular level, while Tier-2 assays confirm an EDCs ability to produce adverse reproductive effects in whole animals. We propose to develop a series of novel sensitive, rapid and inexpensive Tier- 1 bioassays for high-throughput EDC screening. These recombinant cell bioassays will utilize novel hormone responsive reporter genes which respond to EDCs with alterations in expression of easily measured reporter genes. Systems which allow rapid assessment of the ability of a chemical or mixture to simultaneously affect multiple endocrine systems will also be developed. The lack of appropriate high-throughput Tier-1 bioassays and the federal mandate requiring screening of industrial and environmental chemicals for endocrine disruptor activity, indicates the tremendous commercial potential of these assays.